1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and more particularly to a method of forming a gate insulating film on a semiconductor substrate.
2. Description of the Prior Art
There has been a demand for a manufacturing method capable of manufacturing transistors having high-operating speed and high-reliability at good controllability. To increase the operating speed of MIS (Metal Insulator Semiconductor) type transistors, the thickness of the gate insulating film needs to be reduced; however, various problems arise in the course. For example, one of the problems is that boron diffuses from the p-type gate electrode into the substrate. As a method for solving this problem, Japanese Unexamined Patent Publication No. 11-67760 suggests a method in which a gate oxide film is subjected to a thermal treatment process in an atmosphere such as N2O or NO to introduce nitrogen into the gate oxide film. By providing a layer in which nitrogen is contained in the vicinity of the interface of the gate oxide film with the gate electrode at a high concentration, boron diffusion from the gate electrode is suppressed. Regarding a similar method of introducing nitrogen, Japanese Unexamined Patent Publication No. 2-18934 suggests a method using NH3. Additionally in recent years, there has been proposed a method of introducing activated nitrogen formed by a plasma method or the like into the gate oxide film. For example, Japanese Unexamined Patent Publication No. 2000-294550 shows nitridation and oxy-nitridation employing plasma with the use of a planar antenna and microwave, and Japanese Unexamined Patent Publication No. 2000-332009 shows a method of forming a gate oxide film containing nitrogen utilizing generation of stable plasma. In this way, with the rapid advance of thickness reduction of gate oxide films, introduction of nitrogen into a gate oxide film has particularly become a very important issue.
Meanwhile, it has been known that when nitrogen is introduced into a gate oxide film, decrease of carrier mobility is caused and transistor characteristics are degraded if excessive nitrogen that has been introduced into the gate oxide film is present at the interface between the gate oxide film and a semiconductor substrate. To solve this problem, Japanese Unexamined Patent Publication No. 5-218405 proposes a method in which an additional oxide film is formed below the nitride layer by performing a thermal treatment process in an oxygen atmosphere, whereby the nitride layer is kept away from the interface between the gate oxide film and the semiconductor substrate.
What is important in these nitriding methods is to prevent excessive nitrogen that has been introduced into a gate oxide film from reaching the interface between the gate oxide film and the semiconductor substrate. So, it can be said that in order to achieve this, important factors are realization of nitridation using activated nitrogen that is capable of introducing nitrogen into the surface of the gate oxide film, such as represented by plasma nitridation, and “re-oxidation”, in which nitrogen that has reached the interface between the gate oxide film and the semiconductor substrate is detached from the interface of the gate oxide film and the semiconductor substrate.
However, when nitrogen is introduced into a gate oxide film using nitrogen activated by means of a plasma nitridation technique or the like, not all the nitrogen that has been introduced therein takes part in inter-atomic bonding; some part thereof does not take part in inter-atomic bonding and exist in interstices. This is the first time it has been found that a problem arises that such nitrogen relatively freely migrates after nitridation and easily migrates if a thermal treatment process is carried out later. Specifically, as shown in FIG. 1 for example, the nitrogen that was distributed in the vicinity of the surface of the oxide film immediately after nitridation (a) is redistributed due to the heat (about 680 to 760° C.) during the film formation of a poly-silicon gate electrode (b), which is a later process, and is caused to migrate toward the silicon substrate interface. In addition, as shown in FIG. 1, nitrogen is detected in the gate electrode, which is opposite to the silicon substrate interface. Specifically, as shown in FIG. 2, a silicon nitride film 13 is formed by introducing nitrogen after forming a silicon oxide film 12 on a silicon substrate 11. However, if a thermal treatment process is carried out later, nitrogen diffuses also toward the side of the gate electrode 15 at the same time when nitrogen diffuses into the silicon substrate interface, and it reacts with a gate electrode 15 due to the heat during that time, resulting in formation of a gate electrode 14 in which nitrogen is introduced. Since the amount of nitrogen is small in the layer relative to silicon, it does not lead to formation of an insulating film, such as a silicon nitride film that generally exists, but functions as an impurity in a conductor. Because nitrogen serves as an N-type impurity, it contributes to carrier depletion in a p+ gate electrode of a p-MOS transistor, degrading p-MOS transistor's characteristics.
Moreover, this redistribution does not only occur during the above-mentioned formation of poly-silicon film, but it is also under the influence of all the thermal treatment processes that are performed as later processes. FIG. 3 shows the results of the measurement in which the amount of nitrogen in the vicinity of the interface between the gate oxide film and the semiconductor substrate was measured after each of various thermal treatment process step was carried out. Among the items shown as the names of thermal treatment process steps in FIG. 3, “IMMEDIATELY AFTER FILM DEPOSITION” indicates the state prior to the poly-silicon film deposition, and “POLYSILICON” indicates the state subsequent to the poly-silicon film deposition. “OXIDATION” indicates the state after side walls have been formed on the gate electrode composed of poly-silicon. “NITRIDE FILM DEPOSITION” and. “OXIDE FILM DEPOSITION” indicate the states after a nitride film and an oxide film have been formed, respectively, as interlayer insulating films, and “ANNEALING A” and “ANNEALING B” indicate annealing steps for densifying films after depositing the interlayer insulating films, respectively. From FIG. 3, it is understood that the amount of nitrogen increases also due to the various thermal treatment processes that are performed after the poly-silicon film deposition. In addition, some of the process steps did not show the increases, and when attention was directed to process conditions of each of the thermal treatment processes, it was found that the increases did not occur in cases where the temperature is lower than the maximum temperature of the the thermal treatment processes that had been performed before. In other words, it can be said that the final distribution of nitrogen in the gate oxide film is determined by the maximum temperature of all the thermal treatment processes, although in varying degrees. This fact suggests that even if the initial nitrogen distribution is formed ideally using activated nitrogen, there is a possibility that the distribution may be degraded by a later thermal treatment process. It also indicates that the problem arises that, because of fluctuating of nitrogen concentration which greatly affects transistor characteristics, it is difficult to control transistor characteristics.
In addition, it has been found that such nitrogen that easily moves due to thermal treatment processes also moves even at room temperature, though only slightly, and particularly the nitrogen in the vicinity of the surface disappears when placed in the ambient air until the poly-silicon deposition. It is very important to control the amount of such nitrogen because various transistor characteristics fluctuate even with a slight fluctuation of the amount of nitrogen, but practically, this is very difficult.
Meanwhile, the re-oxidization mentioned above is not universal on its own. And in case of very thin gate oxide films with a thermal nitridation using NO or the like, it is not easy to locate the nitrogen that has been introduced into the interface between the gate oxide film and the semiconductor substrate away from the interface to such an extent that its influence becomes small. As will be understood from FIG. 4, the amount of nitrogen (a) in the silicon substrate interface that is redistributed due to the thermal treatment processes subsequent to the plasma nitridation, such as a poly-silicon film deposition, is approximately 1/10 or lower of the amount of nitrogen (b) that is introduced due to NO or the like. In other words, in cases of NO or the like, it becomes necessary to locate the nitride layer sufficiently away from the silicon substrate interface by means of re-oxidization. Assuming that the thickness of the gate insulating film will become 10 Å to 15 Å in the future, the initial oxide film will be 5 Å and a stable film deposition will be impossible for mass production; therefore, this re-oxidization is not realistic. Further, it has been made clear experimentally that, at the re-oxidation temperature of 800 to 900° C. as shown in Japanese Unexamined Patent Publication No. 4-140399, it is impossible to form an oxide film layer (layer for locating the nitride layer away from the interface) at a sufficient thickness at the interface, within a nitrogen-containing oxide film in which nitrogen is introduced at a concentration that is realistically necessary (1 to 20%).
It is a primary object of the present invention to reduce diffusion of excessive nitrogen to the interface between the gate oxide film and the semiconductor substrate caused by the re-distribution of nitrogen, which has been a problem in the prior art, while adopting a nitriding method that is capable of obtaining theoretically desirable nitrogen distributions, and to suppress carrier depletion in the gate electrode and diffusion of nitrogen to the polysilicon interface, which hinders manufacture of high speed transistors, so that high-speed devices can be realized. In particular, an object is to provide a method capable of achieve the foregoing in cases where the thickness of the gate oxide film is very thin, 20 Å or less. In addition, an object is to provide a method to control the amount of nitrogen, which sensitively affects transistor characteristics, and specifically, to suppress its fluctuation due to exposure to the ambient air.